$liste_diff = [];
    
    // - Liste de diffusion
    if(isset($_POST['list']) && $_POST['list']){
        $rs_list = $conn->prepare('SELECT DISTINCT c.mail FROM compte c
        INNER JOIN tl_compte_foyer cf ON cf.idx_compte = c.id_compte
        INNER JOIN utilisateur u ON c.id_compte = u.idx_compte
        INNER JOIN tl_utilisateur_foyer uf ON uf.idx_utilisateur = u.id_utilisateur
        WHERE cf.idx_foyer = ? AND uf.mailing = 1');
        $rs_list->bindParam(1,$_SESSION['foyer']);
        $rs_list->execute();
        foreach( $rs_list as $list){
            array_push($liste_diff,$list[0]);
        }
    }
    if(isset($_POST['address']) && $_POST['address'] != ""){
        foreach ($address as $add){
            array_push($liste_diff,$add);
        }
    }

    // - Liste Admin
    if(isset($_POST['admin']) && $_POST['admin']){
        $rs_admin = $conn->prepare('SELECT DISTINCT c.mail FROM xxx c
            INNER JOIN xxx cf ON cf.xx = c.xx
            INNER JOIN xx u ON c.xx = u.xx
            INNER JOIN xx uf ON uf.xx = u.xx
            WHERE cf.xx = ? AND uf.admin = 1
        ');
        $rs_admin->bindParam(1,$_SESSION['foyer']);
        $rs_admin->execute();
        // $rs_admin->fetch();
        foreach( $rs_admin as $index =>$admin){
            array_push($liste_diff,$admin[0]);
        }
    }

    $liste_diff  = array_unique($liste_diff);

    var_export($liste_diff);

$mail = new PHPMailer(true);

        try {
            
            foreach($liste_diff as $diff){

                //Server settings
                $mail->SMTPDebug = 0;                      //Enable verbose debug output
                $mail->isSMTP();                                            //Send using SMTP
                $mail->Host       = $dmail['smtp'];                         //Set the SMTP server to send through
                $mail->SMTPAuth   = true;                                   //Enable SMTP authentication
                $mail->Username   = $dmail['mail'];                         //SMTP username
                $mail->Password   = $dmail['pass'];                         //SMTP password
                $mail->SMTPSecure = PHPMailer::ENCRYPTION_SMTPS;            //Enable implicit TLS encryption
                $mail->Port       = $dmail['port'];                         //TCP port to connect to; use 587 if you have set `SMTPSecure = PHPMailer::ENCRYPTION_STARTTLS`
                $mail->CharSet    = "UTF-8";
                
                //Recipients
                $mail->setFrom($dmail['mail']);
                $mail->addAddress($diff);     //Add a recipient
                //$mail->addAddress('[email protected]');               //Name is optional
                //$mail->addReplyTo('[email protected]', 'Information');
                //$mail->addCC('[email protected]');

                

                
                // $mail->addBCC($diff);
                
                
                //Attachments
                //$mail->addAttachment('/var/tmp/file.tar.gz');         //Add attachments
                //$mail->addAttachment('/tmp/image.jpg', 'new.jpg');    //Optional name

                //Content
                $mail->isHTML(true);                                  //Set email format to HTML
                $mail->Subject = $objet;
                $mail->Body    = $message;
                $mail->AltBody = 'This is the body in plain text for non-HTML mail clients';

                if($mail->send()){
                    $mail_ok = true;
                }
            }

Vérifie comme dit @arthur et ne remets pas à chaque fois tous les "$mail->..." dans ta boucle, inutiles de les redéclarer. Par contre, laisse " $mail->addAddress($diff);" et fais un "$mail->clearAddresses();" juste après "addAddress" pour pas que le prochain envoi ajoute le mail précédent à la liste des personnes qui vont recevoir le mail.

https://hotexamples.com/examples/-/PHPM ... mples.html

Ton "try/catch" est à mettre dans ton foreach, pas le foreach dans le "try/catch".

"$mail->CharSet = "UTF-8";" est inutile, c'est le charset par défaut, il me semble.